The present invention relates to photolithography used in the microfabrication of semiconductor devices, and, more specifically, to methods of forming a porous silicon film, particularly for use as a pellicle to protect an extreme ultraviolet radiation photomask or reticle.
It is common to employ ultraviolet (UV) radiation in photolithography to transfer a pattern onto an article to be processed, such as a semiconductor wafer. As semiconductor devices continue to be reduced in scale, shorter wavelengths of radiation are preferred. As a result, extreme ultraviolet (EUV) radiation can be used in the microfabrication of semiconductor devices to form components and/or patterns at even smaller scales than more conventional UV photolithography allows, such as features with dimensions in the order of 20 nanometers (nm) or smaller. EUV is often regarded as including wavelengths of from about 4 nm to about 40 nm, which roughly corresponds to frequencies of about 75 petaHertz (PHz) to about 7.5 PHz and/or photon energies of from about 310 electron-volts (eV) to about 31 eV. However, EUV light is highly absorbed by most known materials, which can result in flaws in pattern transfer should undesired particles lay between the EUV radiation source and the photomask and/or the article to be patterned. This is exaggerated in EUV photolithography since radiation is reflected off the photomask instead of shone from behind the photomask, potentially creating twice as much risk of a particle entering the path of the radiation. To reduce introduction of such particles, a shield or the like called a pellicle can be placed in front of a photomask, but such a pellicle can result in significant reduction in EUV radiation transmittance to the photomask and article.
One approach to avoid EUV radiation transmittance reduction is to create a thermal gradient over an article to be processed and/or the photomask, which can avoid use of a pellicle at all. The article, photomask, and/or support structure is heated so that convection currents can form and flow away from the photomask, carrying undesirable particles away, as well. However, this approach may not be as effective in processes in which vacuum is employed. In addition, should gas or particles surround the photomask, the convection currents could draw particles to the photomask.
A related approach is to place an electrostatic charge on the photomask, the article to be processed, and/or support structure. By using a charge similar to that possessed by undesirable particles, the particles can be repelled. This approach may not be as effective where particles of mixed charges are present and/or where a charge could damage the photomask, support structure, and/or article.
Another approach is to use a pellicle, but to remove the pellicle just before exposing the photomask. The photomask in this example is typically in a box sealed by a removable pellicle. The box is inserted into a vacuum chamber in which the article is to be exposed to EUV radiation, the pellicle is removed, the article is exposed, the pellicle is replaced, and the box is withdrawn. This approach may not be effective against particles produced from rubbing parts of the box or elsewhere in the vacuum chamber during exposure.
A further approach is to form a permanent pellicle on the box from a highly EUV transmissive material. For example, a very thin layer or film of a material, such as silicon, can be highly transmissive of EUV radiation and used as a pellicle to cover a photomask box. Similarly, an aerogel membrane of a material, such as silicon, can be formed as a highly EUV transmissive pellicle for a photomask box. Such pellicles typically are very thin and/or of very low density, however, which can result in undesirable deflection and/or breakage.